Depending upon the collection efficiency with which particles having predetermined particle sizes are collected, air filters are classified as, for example, ULPA (ultra low penetration air) filters, HEPA (high efficiency particulate air) filters, or medium efficiency particulate air filters. These air filters are used in different applications in accordance with the performances thereof.
In order to make the effective area, which is a collection allowing area, as wide as possible, these air filters are sometimes used in a pleated state.
As a structure that maintains the pleats of air filters in this way, for example, the following structure of an air filter described in Patent Literature 1 (Japanese Unexamined Patent Application Publication No. 2013-52321) is proposed. In this structure, concave portions and convex portions are formed by embossing surfaces of an air-filtering medium, the intervals between facing surfaces are maintained by the concave portions and convex portions, and the facing surfaces are maintained in a separated state even during use.